This invention relates to a drive device for control rod drive mechanisms using electric motor drive in an atomic power plant.
With the revolution in reactor control technology in recent years, for control rod drive mechanisms in atomic power plants, use has come to be made of control rod drive mechanisms in which the position of the control rod is controlled using an electric motor, rather than control rod drive mechanisms as conventionally employed, in which insertion and withdrawal operation was performed by water pressure.
In such control rod drive mechanisms, control of the direction of drive (insertion or withdrawal) of the control rod and/or the drive speed and position of the control rod can be effected by changing the voltage, frequency and drive time of the electric motor by operating switching elements in an inverter power source constituting the power source of the electric motor that drives the control rod.
However, since, in such a control rod drive mechanism, the electric motor is controlled using an inverter power source, techniques for suppressing the adverse effects of noise generated from this inverter power source have become necessary.
A conventional drive unit for a control rod drive mechanism comprises, as shown by the block diagram of FIG. 1, 205 control rod drive mechanisms 1, 205 inverter power sources 2 corresponding to the drive mechanisms, and 205 inverter controllers 3, as well as a control device 4 and a man-machine device 5.
The plurality of control rod drive mechanisms 1 installed at the bottom of the reactor of the atomic power plant deliver output to control unit 4 in the form of control rod position signals S1 that indicate the positions of the control rods, not shown, in the reactor.
When the control rods are driven, man-machine device 5 is used to select (1) a control rod selection mode (individual or ganged), (2) a control rod drive mode (step, notch, or continuous), or (3) a control rod insertion/withdrawal mode. The information regarding which selection has been made is output to control unit 4 in the form of drive information S2 indicating the target position obtained by calculation in accordance with the selection that was made.
(1) The control rod selection mode is the mode that selects which control rods within the reactor are to be driven, and may be specified as either individual mode or ganged mode. Of these, the individual mode is a mode that is employed for surveillance of the control rods. In this mode, the control rods can be driven one at a time.
Also, in the case where movement of the control rods is automated, the control rods are driven as groups of control rods comprising, for example, from 2 to a maximum of 26 control rods. This is called the ganged mode. When driving is effected by this ganged mode, up to a maximum of 26 inverter power sources 2 must be driven concurrently.
(2) The control rod drive mode is the mode for determining how the control rods are to be driven, and may be specified as a step mode, a notch mode or a continuous mode.
Step mode is a mode in which control rods are only driven through a fixed distance. This is employed when making fine adjustments, etc to the output of the reactor. The switching elements of inverter power sources 2 are turned ON or OFF so as to move control rod drive mechanisms 1 only through the width of the step.
Notch mode is a mode in which driving is effected through a distance of four times the step. The switching elements of the inverter power sources 2 are turned ON and OFF so as to move the control rod drive mechanisms 1 through four times the step width.
Continuous mode is a mode in which the control rods are driven continuously up to a target position that is input through man-machine device 5. Inverter power sources 2 output voltage continuously until the control rods reach the target position, whereupon their operation is stopped.
(3) Control rod insertion/withdrawal mode is the mode for specifying whether the control rods are to be inserted or withdrawn. The switching elements of inverter power sources 2 are turned ON and OFF to give output voltages of opposite phase for insertion and withdrawal.
In accordance with the control rod selection mode specified in the drive information S2 that is input from man-machine device 5, control unit 4 outputs the data of the control rod drive mode and control rod insertion/withdrawal mode as an inverter control signal S3 supplied to inverter controller 3 of the control rod drive mechanism 1 that effects drive.
Also, depending on the control rod current-position signal S1 from the control rod drive mechanism 1 that is being driven and in the control rod drive mode, the target position to be reached by the control rod drive mechanism 1 that is driving the control rod is calculated, and the output of inverter control signal S3 is continued until the control rod drive mechanism 1 reaches this target position.
Inverter controller 3 determines the direction of rotation of the motor in accordance with the control rod insertion/withdrawal mode of inverter control signal S3 that is input from control unit 4. Also, inverter controller 3 determines the motor drive time and voltage and frequency that are output by inverter power source 2 according to the control rod drive mode and outputs to inverter drive source 2 as inverter drive signal S4 the changeover timing information of the switching elements in the output unit of inverter power source 2.
Inverter power source 2 supplies the power specified by control rod drive mechanism drive signal S5, in accordance with the control rod drive mode and the control rod insertion/withdrawal mode contained in drive information S2, to control rod drive mechanism 1 under the control of inverter drive signal S4 from inverter controller 3.
Control rod drive mechanism 1 is fed with power from inverter power source 2, and the control rods are driven as long as this power is supplied.
However, when reactor scram occurs, the control rods in control rod drive mechanisms 1 are temporarily separated from the motors and inserted at high speed by water pressure, in response to a full-insertion drive command from the reactor emergency shut-down system.
As a backup system after the control rods of control rod drive mechanisms 1 have been fully inserted by water pressure, all 205 control rod drive mechanisms 1 are driven to a fully inserted position by operation of the electric motors in response to said full-insertion drive command.
Since one inverter power source 2 is provided for each control rod drive mechanism 1, when there are 205 inverter power sources 2, an enormous number of control rod drive devices, i.e., inverter power source 2, and inverter controllers 3, of control rod drive mechanisms 1 are required.
Also, in drive control of the control rods, even in the case of automatic control during reactor operation using a computer, the maximum number of ganged groups which can be operated without driving a plurality of control rods at the same time is 26. Furthermore, in emergency insertion such as reactor scram, the control rods are inserted by a water pressure unit, so there is no need for the motor drive using inverter power sources 2 to be able to actuate all the control rods urgently and simultaneously.
For the above two reasons, the provision of one inverter power source 2 for each control rod drive mechanism 1 means that a large number of inverter power sources 2 are required and inverter power sources 2 generate a lot of noise. A further problem is that equipment cost is increased by the fact that the devices and wiring etc. are complicated, so that a lot of maintenance is required.